Living body information measurement devices are known which can measure living body information such as a pulse rate and a blood pressure using information that is detected by a pressure sensor(s) in a state that the pressure sensor(s) is in direct contact with a living body part where an artery such as a wrist radius artery runs (refer to JP-2004-113368-A, JP-H02-261421-A, JP-H07-124130-A, JP-H01-242031-A and JP-H01-288228-A, for example).
In the blood pressure measurement device disclosed in JP-2004-113368-A, blood pressure values in a part that is different from a living body part to which a pressure sensor is brought into contact are calculated using a cuff and calibration data is generated using the calculated blood pressure values. Blood pressure values are calculated for each beat by calibrating a pressure pulse wave detected by the pressure sensor using the calibration data.
JP-H02-261421-A, JP-H07-124130-A and JP-H01-242031-A disclose blood pressure measurement devices which measure a blood pressure for each beat without using a cuff, that is, using only information detected by a pressure sensor that is set in contact with a wrist.
JP-H01-288228-A discloses a pulse wave detection device which detects pulse waves by pressing, toward an artery, a pressing surface that is formed with plural pressure sensor arrays arranged in a direction in which the artery runs. In this pulse wave detection device, whether the pressing surface is parallel with an artery is judged by comparing pressure pulse waves that are obtained by the plural pressure sensor arrays in a state that the pressing surface is pressed toward an artery. If the pressing surface is not parallel with the artery, posture control is performed that the pressing surface is inclined toward a direction in which the artery runs and thereby setting the pressing surface parallel with the artery.
As disclosed in JP-H02-261421-A, JP-H07-124130-A and JP-H01-242031-A, measuring a blood pressure using only information detected by the pressure sensor that is set in contact with a living body makes it possible to miniaturize the device. The devices disclosed in JP-H02-261421-A and JP-H07-124130-A are such that a blood pressure is calculated by calibrating or correcting a pressure pulse wave that is detected by the pressure sensor in a state that the pressure sensor is pressed by an optimum pressing force.
To determine the optimum pressing force, it is necessary to execute a process in which the pressing force of the pressure sensor is varied continuously. A blood pressure calculation is enabled only after this process is finished and a state that the pressing force of the pressure sensor is controlled to an optimum pressing force is established.
As a result, in these devices, it is necessary to execute processes of pressing the pressure sensor against a living body part, canceling the pressing, and pressing the pressure sensor in such a manner that its pressing force is set equal to the optimum pressing force. Repetition of these processes is troublesome to the user.
On the other hand, in the device disclosed in JP-H01-242031-A, blood pressure values can be calculated on the basis of a variation of the amplitude of a pressure pulse wave that is detected in a process that the pressing force of a pressure sensor is varied continuously. As a result, a blood pressure can be measured in a short time without causing a user to feel troublesome.
However, in the process that the pressing force of the pressure sensor is varied continuously, the position of the artery being pressed may vary. In the method disclosed in JP-H01-242031-A, such a variation may disable an accurate measurement of a blood pressure. The device disclosed in JP-H01-242031-A measures a blood pressure for each beat. A similar problem arises also in other types of devices that measure a blood pressure while a pressure sensor is pressed against a local part.
In the device disclosed in JP-H01-288228-A, posture control is performed on the pressing surface by comparing pressure pulse waves that are output from the respective pressure sensor arrays in a state that the pressing surface is pressed toward an artery with a prescribed pressure. The posture control is not performed taking into consideration a relative positional relationship between the pressing surface and the artery in a process that the pressing force of the pressing surface toward the artery is varied.